Numerical Studies On The Influence Of Injection Strategy On Gasoline/Diesel Dual Fuel Highly Premixed Charge Combustion

Gasoline/diesel dual fuel Highly Premixed Charge Compression combustion mode is able to control in-cylinder reactivity and fuel concentration distribution with high flexibility, it has been widely concerned by world-wide for its potential in high efficiency and clean combustion. In-cylinder diesel fuel injection strategy plays significant important role in its combustion process control. This paper conducted numerical study on influence of injection strategy on combustion and emission characteristics in a gasoline/diesel dual fuel HPCC engine.Influence of in-cylinder diesel fuel injection timing of HPCC has been performed, while keeping approximately the same operating condition, such as fuel mass, CA50 timing, IMEP and EGR. The results indicated that in-cylinder free radicals originate from n-heptane is the key factor that controls the ignition and combustion process: late-injection produces local regions with high n-heptane concentration and increases the reactivity, results in earlier ignition; early-injection of n-heptane increases the amount of free radicals at ignition timing, results in higher peak pressure and heat release rate. L-HPCC mode produces higher local temperature and more NOx emission compared to that of the E-HPCC; although the local regions promote the soot formation, however, the higher combustion temperature could also enhance the soot oxidation process, which makes soot emission lower than that of the E-HPCC mode.A numerical study on the combustion characteristics of single-injection and double –injection gasoline/diesel HPCC combustion was also conducted. The results showed that the nature of the gasoline/diesel dual fuel HPCC combustion is the interaction between direct injection of diesel to increase fuel reactivity and concentration stratification. Single-injection strategy can reduce PRR(pressure rise rate) by producing more reactive radicals before the high temperature reaction. With double-injection, high n-heptane concentration(higher reactivity) near nozzle is the key reason for the advancement of the high temperature reactions. In double-injection strategy, reduction of the in-cylinder reactivity is the key reason to reduce the heat release rate(HRR)(combustion rate), advanced first injection timing allows fuel to have longer mixing time to suppress the production of reactivity radicals, so that low temperature reaction was suppressed, which enhances the two-stage heat release behavior; in the later first injection timing case, low temperature heat release by the first injected fuel promotes the decomposition and oxidation of the later injected fuel, thus in-cylinder reactivity was enhanced, results in advanced high temperature reaction-higher pressure rise rate.Effects of premixed gasoline ratio on combustion process and emissions of gasoline/diesel dual fuel HPCC was investigated by numerical simulation based on injection timing. The results indicated that at early injection timing, the amount of n-heptane injection mass is the key factor that controls the in-cylinder reactivity, thus influence combustion parameters such as ignition delay and peak pressure. At late injection timing, high n-heptane concentration increases local reactivity, decreases low temperature combustion(LTC) duration, makes in-cylinder ignition happened quickly after n-heptane injection, reduces sensitivity of premixed gasoline ratio to ignition delay. With late injection, the inhibition effect of local low oxygen concentration to n-heptane elementary reaction is enhanced by n-heptane injection ratio, result in two-stage consumption of n-heptane at G0 case, which suppresses soot formation rate and retards peak soot timing; however, higher local reactivity at G0 case increases local peak temperature region, promotes NOx formation during combustion process.